During the assembly of a circuit, a card is fed into different machines. The first machine is a solder paste printer which prints the solder paste pattern and the second machine is a component pick and place machine which positions and fixes the components into the solder paste. During the conveyor transport and processing of the card, it experiences forces, which distort the surface of the card. For example, the card is typically fed into the machines on "rail" conveyors. These conveyors have belts which touch the outside edges of the card. Because the board spans from one rail to the other, many times the weight of the card alone will cause it to flex. The assembly processes (namely printing and component placement) require a flat, rigid circuit surface--any flexing and distortion causes quality problems and defects. The costs of repairing or reprocessing are very high. These production lines generally run as fast as possible, so it is very costly to implement custom tooling to support the boards.
Most assembly machines are designed for thick, rigid boards with components on one side. Electronic miniaturization for notebook computers and portable devices has forced manufacturers to reduce the thickness of the boards. The board sizes have shrunk as well. When the board size is reduced, manufacturers will process several cards as one "panel". In panel form, the card rigidity is reduced further because the fabrication process adds "routed" slots which allow the manufacturer to "break" the finished cards out of the panel after assembly.
The thinner cards and the routed slots of the panels have reduced the rigidity to a point where the cards are extremely flexible and the quality of the finished card is severely affected.
In the surface mount printing process, cards move under a stencil foil (which has a matching pattern) and solder paste is deposited onto the card. In the printing process there is a squeegee which must press onto the thin stencil foil mask. The force can exceed one pound per inch of squeegee. The printing process requires that the squeegee force be transmitted to the circuit card. The goal is for the squeegee to push the stencil against the card, and then to slide solder paste across the top of the stencil to complete the printing process. Sufficient pressure is required to insure a good gasket between the stencil and card, and to prevent solder paste from extruding under the squeegee.
To achieve good quality with the printing process just described, the card must be supported.
There are two types of printing machines in use today: Batch machines with a tooling plate that shuttles the card in and out of the printing area, and Automatic In-Line machines which feed the card on rail conveyors into the print area, where an elevating tooling plate rises to make contact with the card.
For purposes of this disclosure, the processing of the top side and bottom side of the card are referred to as "side 1" and "side 2" respectively. The side 2 assembly process is different from the side 1 assembly process because the circuit card must be handled with components already assembled to side 1. In this mode, neither the batch machines nor the in-line machines can support the circuit card without the use of customized tooling. Because the board is not flat on the bottom, a tooling plate must be machined out of solid aluminum (for stiffness). This requires complicated cut out patterns to avoid contact and damage to the components on the underside of the circuit card. These plates add engineering time and cost to the production of the thin circuit cards. (For side 1 production, the circuit card is essentially flat on the bottom. Therefore, simply placing the card on a flat piece of aluminum is sufficient for processing). FIG. 1 shows a prior art custom tooling plate for an in-line printing system.
It should be noted that when processing the circuit card, a tooling plate provides three functions: 1) it supports the assembly and provides "nesting" for underside components, 2) it registers the location of the circuit card using tooling pins, and 3) it provides some form of hold-down technique, such as vacuum or edge clips. This hold-down is necessary for boards that have warpage, and do not lay flat in the tooling plate nest.
An alternative prior art tooling method using support pins, see FIG. 2, is sometimes employed with in-line automatic printers and placement equipment. This method feeds the card into the print area. Once positioned, an array of support pins is brought into contact with the circuit card's bottom side. This method is more common, but less effective than the machined plate method. It has a lower cost because the pins may be repositioned for use on other cards (rather than having to machine a new tooling nest). However, for cards less than 0.059" thick the pins are not spaced closely enough to prevent flexing. It would be difficult to modify this technology to handle the thin cards. Finally, this method involves lengthy setup times, each pin must be checked for x-y location, and must be positioned in the z-axis to accommodate the height of any components the pin is touching.
In summary, both the custom tooling plate and the pin-array method involve lengthy setup times. Although the custom machined plate is effective, the cost is very high ($500-$1500 each plate), and the plates are not reusable. The pin-array method does not work with thin cards. The flexing reduces the print quality to unacceptable levels.
The problems caused by the flexing of the cards, although described in reference to the printing step, are also common to the step of component alignment and placement.
In summary with these prior art techniques the steps involved making a typical side 2 custom tooling nest are first, the component locations and board outline for side 1 are plotted out on a drawing with dimensions. Then an engineer carefully designs cutout regions based on the side 1 component information. This engineering effort involves deciding what radius tool to be used for the component cutouts, and selecting the clearances around parts. Further, the engineer must locate tooling pins, and hold down elements so the circuit card is registered accurately, and it is pulled down flat. This design process can take as long as 3 days. The ability to produce a perfect tooling plate the first time is limited. This fixture is designed with respect to the engineering drawings for the PC card and the components. However, components for side 1 may actually have a variation in size and shape. These variations cause interference and sometimes a second fixture has to be designed. This design time extends the setup of the manufacturing process beyond what is reasonable. After the design is complete, the engineering information is passed to a machine shop which mills the aluminum plate to specifications. This step adds additional time.
A third alternative prior art `tooling` method, FIG. 3, utilizing a moldable foam material (or viscous curable liquid) is disclosed in U.S. Pat. No. 5,054,193. This method teaches creating a recessed tooling plate by heating a heat-moldable foam, pressing the side 1 of the card into the foam and then cooling the foam to retain the shape of the PC card side 1. An alternative embodiment suggests a curable viscous material and a membrane to separate the material from the PC card.
There are several problems associated with the disclosed process and fixture. There is no teaching of how to handle and support the PC card during testing. A nest formed with a molding process is inadequate without an effective PC card support and preparation stage. This PC card support is inadequate because the non-component side of the PC card is not perfectly smooth. There are inherently small bumps such as the printed wiring, printed wiring bumps, plating bumps or other surface profile features which make the non-component side non-flat. If a flat plate is used to press the card into the foam, then the card will flex on the points of contact with the plate, producing a non-flat support fixture. This flexure is a deviation from true flatness. The bumps will be randomly located from one card to another so this fixture forming process is sensitive to these bumps. The fixture will have a planar error which would affect overall support.
The bumps on the non-component side 1 occupy a very small percentage of area (under 5%) while the components on side 2 occupy more than 60% of the area.
As described hereinafter, this bump and flexing problem does not occur with the invention disclosed herein because a thin compliant layer is placed between the PC card and the support plate. Since the bumps occupy only a small percentage of area, they depress into the compliant member until the PC card surface touches. Thus, this brings the PC card parallel with the support plate.
A second drawback with the '193 patent is the use of the foam as a "spring" to press the PC card back to the support plate. The pressure created by the compliant foam will be proportional to the displacement of the foam. Therefore, the back pressure will be uneven.
The invention described herein in one aspect comprises the use of a viscous casting material with a very distinct PC card preparation sequence to form a nested support fixture. Another aspect of the invention is an improved molded (tooling plate) support fixture. The support fixture of this invention produces planar support even when the PC card has inherent warpage and side 2 bumps.
In one embodiment of the invention, the support fixture is formed by a combination of a vacuum platen, a compliant member on the platen which member supports a PC card, side 2, an impermeable stretchable membrane and a perimeter wall to define a well.
The PC card is drawn down flat into the compliant member which lies on the flat vacuum platen. This "drawing" action is achieved by use of vacuum and the impermeable, stretchable membrane.
The invention described in this disclosure places a compliant member between the flat surface onto which the PC card rests. This compliant member serves two unique functions. This complaint layer (to be described in detail later) allows the bumps to sink in, while the flat surface of the PC card rests flush with the compliant layer.
When the membrane is drawn fully down onto the PC card, it pulls and flattens the PC card and defines its position and planarity in preparation for casting. The bumps will have depressed into the compliant member. The membrane is actively utilized as a mechanical locking mechanism. A perimeter wall is formed and a very accurate three dimensional mold results.
Stated otherwise this invention comprises a flexible membrane which is used as a vacuum seal to pull the PC card flat and to press it into the thin compliant member. The complaint member allows bumps on the non-component side to depress in, leveling the PC card to a true flat plane. The compliant member distributes the vacuum over the full area of the complaint member. If this complaint member were non-porous, the vacuum would not distribute evenly, and the forces holding the card in place would be non-uniform. The compliant member maintains equal, distributed PC card support and pressure by use of a central vacuum and the membrane which sandwiches the PC card and compliant member.
The invention described is insensitive to sample PC card warpage, and side 2 bumps, both of which occur randomly.
The present invention combines the vacuum, the compliant-porous member which distributes the vacuum, and the membrane to force the membrane into a three-dimensional contour of the sample PC card without wrinkles. This is accomplished because the vacuum causes the forces to be distributed everywhere over the membrane area, pulling and eliminating wrinkles.
Simply placing a three-dimensional detailed object (PC card) into a viscous liquid or into a foam with a membrane separating them will involve many regions of entrapped air, or in the case of the foam, entrapped and displaced air. These air bubbles will become incorporated into the finished fixture and will result in a lack of support.
The present invention is further distinguished from the prior art in the ability to seal objects and create accurate three dimensional geometries of these objects from the surface of the component side of the PC card.
In a preferred embodiment of the invention, in forming a support fixture, a rectangular frame and a cylindrical plug are placed under the membrane. The frame is used to create a mold and the cylindrical plug is used to create a recess to allow easy finger access to remove the PC card from the fixture. By placing the plug and frame under the membrane, and drawing vacuum, the invention provides a "locking and fixing" capability for any geometric elements that are required in the finished support fixture. Alternatively, geometric elements may be anchored into the mold material, for permanent use, or for later removal, to create elements which are not separated from the mold by the membrane.
A casting material is poured into the mold (the region defined by the rectangular frame and PC card surface both of which are covered by the membrane). The membrane serves to hold the elements in place during the casting step and also serves to prevent contamination of the components on the PC board. The thickness of the membrane serves to enlarge the recesses in the finished fixture. The thickness is set to a minimum (0.01 inches). This thickness enlarges the recesses, while covering over small delicate areas which, if uncovered, would result in a fixture which may damage delicate areas.
Other elements such as tooling pins or vacuum plugs may be anchored into the casting material directly by positioning them before or after the casting material is poured. These elements may be placed outside the membrane or the membrane may be cut and the elements placed directly in contact with the PC card. A unique aspect of this system is the ability to excise the membrane to anchor elements so they are directly in contact with the PC card. When the membrane is fully evacuated, it may be excised on smooth areas. The excising does not break the vacuum because the open area remains sealed due to the flat contact between the membrane and the surface of the card.
The casting material is poured to overfill the mold. A backing plate is pressed onto the overfilled mold. The plate squeezes excess casting material out through large back holes and around the edges of the frame. The large holes in the back plate serve three functions. They provide a standard pattern within which vacuum feed holes are placed. This enables a single manifold vacuum block to accommodate many cast fixtures. They provide a locking feature to keep the hardened casting material attached to the back plate. Thus, they serve to "lock" the back plate to the casting material which prevents the casting material from shrinking excessively. They provide a `feed" point where excess casting material is left to feed into the main mold as the curing casting material (fixture) shrinks. Without these feed locations, the curing process may create voids due to the shrinkage.
The invention described herein is applicable to the handling of the circuit card through both process steps: printing and component placement.
The fixture described in this invention utilizes a finished side 1 assembly and a casting process to produce an accurate recessed nest in the casting. The completed support fixture is comprised of the nested casting, a backing plate, and vacuum and/or tooling elements. The process for making the fixture involves unique equipment and a sequence of operations, combined with the finished side 1 assembly.